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Usually when we measure current, we have to break the circuit and place the ammeter in series.

However, if we want to make it easy to measure current at various points in a permanent circuit(for whatever reason), can we add very low resistances in the circuit at those points,, measure the voltage across them instead and use Ohm's law to find the current?

If the effect of those resistors is negligible, and they have no effect on the functioning of the circuit, is doing this ok?

edit: I can't think of a suitable title for this question, help on that would be appreciated as well:)

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    \$\begingroup\$ Not sure of the proper name in English of the device, but for measuring AC currents you have special devices to clip onto a wire and measure the current by its magnetic field. These things exist in various sizes, for in circuit use and 'clip on' for use with a multimeter. \$\endgroup\$ – jippie Jul 3 '12 at 7:47
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    \$\begingroup\$ @jippie - they are called a current clamp and some can measure DC as well (e.g. Hall effect versions) \$\endgroup\$ – Oli Glaser Jul 3 '12 at 7:52
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Yes, that's a good way of doing it, if the effect of the measurement resistors is indeed negligible. That could be a problem with low voltages: you want to have a resistance high enough to measure at least several tens of mV, but if that branch has only 100 mV between nodes then the resistor may affect the circuit's operation.

But often you don't need a shunt resistor, if you already have a resistor in the branch you can measure across that.

Note that for AC measurements the current you measure will always be in phase with the branch's voltage, and that may be quite different from the current's real phase. For instance, if you want to measure the current through a capacitor that will be 90° out of phase with the voltage, but what you measure across the resistor will have 0° phase shift.

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  • \$\begingroup\$ Thanks for your help (again!). I have one more question on this. Is calling this resistor a shunt resistor correct? \$\endgroup\$ – Ammar Jul 3 '12 at 7:50
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    \$\begingroup\$ @Ammar - I think so. Or you can always call it a sense resistor. \$\endgroup\$ – stevenvh Jul 3 '12 at 7:54

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